Common mode filters have been known as one of electronic components used for high-frequency circuits. There are mainly two types of common mode filters, one is a winding type and the other is a thin film type. For high frequency circuits, winding-type common mode filters are often used.
A winding-type common mode filter includes a drum core, two winding wires wound around the drum core, and a plurality of terminal electrodes that are each electrically connected to ends of the two windings. The drum core has a pair of flanges and a winding core that connects the pair of flanges. Each of the two windings has a winding portion wound around an outer peripheral surface of the winding core and a lead portion that extends from each end of the winding portion to a tip of the winding.
Each of the two windings is electrically connected with a corresponding terminal electrode at its lead portion. The winding portion is wound around the winding core such that it contacts the outer peripheral surface of the winding core. The winding portion has more than one turn. In order to match a characteristic impedance of the winding, it is desirable that a capacitances generated between two adjacent turns in the winding portion be constant by arranging the turns at a regular interval in the winding portion.
Conventional winding-type common mode filters are disclosed in, for example, Japanese Patent Application Publication No. 2005-56934 (“the '934 Publication”), Japanese Patent Application Publication No. 2012-29210 (“the '210 Publication”), Japanese Patent Application Publication No. 2002-008931 (“the '931 Publication”), and International Publication WO 2008/096487 (“the '487 Publication”). In these conventional common mode filters, terminal electrodes are provided on a lower surface or an upper surface of the flange, and the lead portion of the winding is coupled to a junction portion of a terminal electrode.
It is desirable that the two windings of the common mode filter are formed to have the same length. This is because it is difficult to match the characteristic impedance if the two windings have different lengths.
However, in conventional common mode filters in which the terminal electrodes are provided on one of the lower surface or the upper surface of the flange, the geometrical arrangement of the two windings, in particular, the geometrical arrangement of the lead portions becomes asymmetric with respect to each other, and therefore it is difficult to make the lengths of the two windings equal to each other.
When a general-purpose drum core is used, it is particularly difficult to make the length of the lead portion of one of the two windings equal to the length of the lead portion of the other winding. For example, when a general-purpose drum core is used, to obtain a small inductance design, the number of turns of the winding around the winding core is decreased. Accordingly, referring to FIG. 3 of the '487 Publication, the windings are disproportionally arranged toward one of the flanges.
As described in the '210 Publication, when two windings are bifilar-wound, if a distance between adjacent turns of the two windings is made constant, the two windings can be each formed symmetrically with respect to the center of the winding core of the drum core in the length direction of the winding core. However, as pointed out in paragraph [0006] of the '210 Publication, it is difficult to bifilar-wind the two wires such that the interval between the two windings becomes constant.
FIG. 5 of the '931 Publication discloses a common mode filter in which a concave portion is formed on each end surface of each flange and a terminal electrode is provided in each of the concave portions. In such a configuration, it is possible to arrange the two windings geometrically symmetrical by providing the terminal electrodes on the two opposing side surfaces of each flange.
However, in the common mode filter described in the '931 Publication, the lead portion of each winding is connected to the terminal electrode in a slack state. More specifically, the lead portion of each winding extends in a slight arc from the boundary with the winding portion to the junction portion (the conductive connecting portion E) with the terminal electrode. Therefore, the length of the lead portion changes depending on the looseness of the lead portion. Moreover, when the lead portion is slack, it may lead to variation in the intervals between adjacent turns of the winding portion. In the common mode filter described in the '931 Publication, since the end portions of the windings are bonded to the flat electrode layer by welding or the like, it is difficult to keep the windings tensioned when the windings are bonded to the electrode layer. When the windings are not tensioned at the time of bonding, the windings are jointed to the terminal electrodes such that they remain in a slack state. As described above, in the common mode filter of the '931 Publication, it is difficult to bond the lead and end portions of the winding to the terminal electrodes without any slack.